Crude oil discharging from a production well is usually a mixture of oil and water or of oil, water and gas. It also frequently contains some sand, or similar particulates, which have been swept out of the rock formations by the fluid flow.
Normal practice is to discharge this mixture into a separating vessel where the various phases are separated by gravity into a gas flow, an oil flow, and a water flow which usually contains the sand. On a typical North Sea production platform there may be 20 wells discharging into a single separator. The single phase oil and gas flows discharging from the separator can be accurately metered for the purposes of custody transfer (sale) and taxation. Flow measured in this way measures the combined flow of all the wells together.
In order to best manage the oil field so as to maximise the recoverable reserves and/or minimise the cost it is necessary to know what flows are being produced by each well individually. This is currently achieved by the use of a "test separator" which is a smaller version of the separator previously referred to. Periodically each well will be connected to the test separator so that the flows of each phase can be measured from that well. This has the disadvantages that:
The test separator is heavy and costly;
The results are infrequent;
The manpower costs are high;
It is not suitable for sub-sea production systems.
The industry would like to have a flow meter that can be installed in each oil well flow line that can measure the flows of the three fluid phases, known as a multiphase flow meter, so as to provide continuous field management information. Ideally it should be capable of sub-sea installation. The accuracy does not need to be as high as for single phase metering, and a typical target is .+-.5%
Because of the importance of this topic it is the subject of considerable activity in the U.K. and elsewhere. Various techniques are being examined, but at this time there is no proven commercially available device.
The problem is a complex one, made even more complex by the behaviour of multiphase flow. When two or three fluid phases flow together in a pipe many types of flow pattern and distribution of phases are possible especially when one of the phases is a gas. Different behaviour is seen in horizontal pipes, pipes inclined up, pipes inclined down, and vertical pipes. Surges are common. It is also common for the different phases to travel at substantially different velocities and in particular for a gas phase to have a higher velocity in the pipe than the liquid phases.